TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4391 SUBJECT: GRB051016: analysis of the XMM-Newton observation DATE: 05/12/23 20:17:20 GMT FROM: Andrea De Luca at IASF-CNR,Milano Andrea De Luca (INAF/IASF, Milano) on behalf of a larger collaboration reports: We have analyzed the data from the XMM-Newton observation of GRB051016, discovered by Swift on 2005, October 16 at 05:23:31 UT (Boyd et al., GCN4096). The XMM-Newton observation started on 2005, October 16 at 13:44 UT and lasted for 29.4 ks. As reported by Loiseau and Perez-Martinez (GCN4101), the afterglow of GRB051016 is clearly detected in the EPIC images and its position is fully consistent with the refined Swift/XRT one (Morris et al., GCN3606). Extracting source events from a circle of 25 arcsec radius (containing ~80% of the total counts), the time-averaged, background-subtracted count rate in the 0.5-8 keV range is 0.028+/-0.001 cts/s, 0.0092+/-0.0006 cts/s and 0.0100+/-0.0006 cts/s in the pn, MOS1 and MOS2 cameras, respectively. The afterglow is seen to fade along the XMM-Newton observation, spanning the time range 30.0-59.4 ks after the GRB. The pn background-subtracted light curve (0.5-8 keV) is well fitted (reduced chi2=1.0, 7 d.o.f.) by a power law decay with index delta=0.9+/-0.3 (90% c.l.). The observed afterglow decay is consistent with the XRT results (Beardmore et al., GCN4100). We extracted the time-averaged spectra and generated ad-hoc response and effective area files for the pn, MOS1 and MOS2 cameras. The spectra were fitted simultaneously. Unless otherwise specified, we quote errors at 90% level for a single interesting parameter. A fit in the energy range 0.5-8 keV with an absorbed power law model yields an unacceptable reduced chi2 of 2.5 for 26 d.o.f., The resulting NH=(3.0+/-0.2)x10^21 cm^-2 is somewhat higher than the expected Galactic value in the burst direction (NH=1.1x10^21 cm^-2, Dickey & Lockman, 1990); the best fitting power law photon index is Gamma=2.6+/-0.3. A better fit may be obtained fixing the NH to the Galactic value and adding a redshifted neutral absorber. This yields a reduced chi2 of 1.74 for 25 d.o.f. The intrinsic gas column density and the redshift are not well constrained owing to their strong correlation. The best fit values are: intrinsic NH=6.7x10^22 cm^-2, z=4.1. At 90% confidence level for 2 parameters, the intrinsic NH is larger than 5x10^21 cm^-2 and the redshift is larger than 0.8. The best fit value of the power law photon index is Gamma=2.5+/-0.2. The observed flux (0.5-8 keV) is of 7.6x10^-14 erg cm^-2 s^-1, corresponding to an unabsorbed flux of 1.1x10^-13 erg cm^-2 s^-1.